To determine the viscosity of a liquid flowing in a pipe, use is frequently made of meters which, using a vibratory transducer, comprising a flow tube communicating with the pipe, and control and evaluation electronics connected thereto, induce shear or friction forces in the fluid and derive therefrom a measurement signal representing the viscosity.
U.S. Pat. Nos. 4,524,610, 5,253,533, 6,006,609, or EP-A 1 158 289, for example, disclose in-line viscometers, i.e., viscometers connectable into a fluid-conducting pipe, with a vibratory transducer which responds to the viscosity of the fluid flowing in the pipe and comprises:                a single straight flow tube for conducting the fluid which vibrates in operation and communicates with the pipe via an inlet tube section and an outlet tube section;        an excitation assembly which in operation excites at least part of the flow tube into torsional vibrations about an axis of vibration aligned with the flow tube; and        a sensor arrangement for locally sensing vibrations of the flow tube.        
As is well known, straight flow tubes, when excited into torsional vibrations about an axis aligned with the flow tube, cause shear forces to be produced in the fluid flowing through the tube, whereby vibrational energy is removed from the torsional vibrations and dissipated to the fluid. This results in the torsional vibrations of the flow tube being damped, so that additional excitation energy must be supplied to the flow tube to maintain those vibrations. The applied excitation energy can be measured in a suitable manner, and the viscosity of the fluid can be derived therefrom.
In operation, the flow tubes of such transducers, which are used in in-line viscometers, for example, are generally excited at an instantaneous resonance frequency of a fundamental torsional mode, particularly with the vibration amplitude maintained at a constant value.
It is also common practice to excite the flow tubes for viscosity measurements, simultaneously or alternately with the torsional mode, into flexural vibrations, preferably at a resonance frequency of a fundamental flexural mode, see also the above referred to U.S. Pat. No. 4,524,610. Since this flexural resonance frequency is also dependent on the instantaneous density of the fluid in particular, such meters can also be used to measure the density of fluids flowing in pipes.
Compared with the use of bent flow tubes for viscosity measurements, the use of straight flow tubes vibrating in the manner described above, as is well known, has the advantage that shear forces are induced in the fluid over virtually the entire length of the flow tube, particularly with a great depth of penetration in the radial direction, so that very high sensitivity of the transducer to the viscosity to be measured can be achieved. Another advantage of straight flow tubes is that they can be drained residue-free with a high degree of reliability in virtually any position of installation, particularly after a cleaning operation performed in-line. Furthermore, such flow tubes are much easier and, consequently, less expensive to manufacture than, for example, an omega-shaped or helically bent flow tube.
An essential disadvantage of the prior art transducers lies in the fact that in operation, torsional vibrations can be transmitted from the transducer via the flow tube and any transducer case that may be present to the connected pipe. This, in turn, may result in a zero shift and, thus, in measurement inaccuracies. Furthermore, the loss of vibrational energy to the transducer's environment may result in a substantial deterioration of efficiency and possibly also in a degradation of the signal-to-noise ratio in the measurement signal.